4.3 should be a standalone engine harness...and yes, it will require the GM ECM in order to work. Not sure offhand tho who would have harness adapters so you can keep your Toy instrument panel working...

ECM is for your motor efi control . The truck electrical does not need it. If your 4.3 is carburated then it does not need a computer. Yes you can hook up all your gauges. You need to buy a factory Toyota electrical manual for your truck. check out the guys at Downey off road or Northwest Off Road Specialties, they have done this conversion before